Multi color detector

ABSTRACT

A heterostructure or multilayer semiconductor structure having lattice matched layers with different bandgaps is grown by MOCVD. More specifically, a wide bandgap material such as AlInSb or GaInSb is grown on a substrate to form a lower-contact layer. An n-type active layer is lattice matched to the lower contact layer. The active layer should be of a narrow bandgap material, such as InAsSb, InTlSb, InBiSb, or InBiAsSb. A p-type upper contact layer is then grown on the active layer and a multi-color infrared photodetector has been fabricated.

[0001] This invention is made with government support under Contract No.DARPA/ONR-N-00014-97-1-0799. The government has certain rights in theinvention.

FIELD OF THE INVENTION

[0002] This invention relates to semiconductor III-V alloy compounds, aswell as to a method of making III-V alloy compounds for use in two colordetectors.

BACKGROUND OF THE INVENTION

[0003] Infrared (IR) detection is used in many military and commercialapplications such as pollution monitoring, night vision, missiletracking, and seeker-tracer systems. The detection of infraredelectromagnetic radiation can be accomplished by utilizing eitherthermal or photon detectors. Photon detectors use narrow bandgapsemiconductors, with carriers being generated through the excitement ofelectron and holes by incident light with energy higher than that of thematerial bandgap.

[0004] Most established infrared imaging systems are single colordetectors, meaning that the response of the detector is designed tocover a single region of the IR spectrum. Single color detection issuitable for many applications, but is not adequate for accuratetemperature determination or for reliable object identification ofobjects with unknown emissivities. Two-color detectors are detectorswith photoresponse in two separate spectral regions which can eliminatethe need for knowing specific object emissivities.

[0005] Existing technologies for two-color detection utilize stackeddiodes with different active regions which responds to differentwavelengths of light, such as superlattices with different wellthicknesses and/or band offsets, or by using compound semiconductormaterials with different bandgaps. The former method is rather complex,requiring accurate modeling and very large numbers of layers in thestructure, and the latter can have the problem of lattice mismatchbecause of the requirement for different bandgap active regions. Theactive region could be a ternary or quaternary material, or asuperlattice structure. In either case, lattice matching is veryimportant for higher performance detectors, and the fabrication is quitecomplex. The device is a three terminal device which also addscomplexity to the circuitry used to read out the detector electricalsignal because of the need for three connections to every pixel.

SUMMARY OF THE INVENTION

[0006] These and other objections are attained by the subject inventionwherein a heterostructure having lattice matched layers with differentbandgaps is grown by MOCVD or like process. More specifically, a widebandgap material such as AlInSb or GaInSb is grown on a substrate toform a lower-contact layer. An n-type active layer is lattice matched tothe lower contact layer and doped. The active layer is a narrow bandgapmaterial, such as InAsSb. A p-type upper contact layer is then grown onthe active layer doped and the structure is annealed.

[0007] An object, therefore, of the subject invention is a two-colordetector.

[0008] A further object of the subject invention is a two contact devicewith a narrow bandgap in an active layer.

DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a cross-section of a two-color detector structureaccording to the subject invention.

[0010]FIG. 2 is a graph showing the photoresponse from the two colorphotodiode of the subject invention.

DETAILED DESCRIPTION OF THE INVENTION

[0011] The reactor and associated gas-distribution scheme used hereinare substantially as described in U.S. Pat. No. 5,384,151. The systemcomprises a cooled quartz reaction tube pumped by a high-capacityroughing pump (120 hr-1) to a vacuum between 7 and 760 Torr. Thesubstrate was mounted on a pyrolytically coated graphite susceptor thatwas heated by rf induction. The pressure inside the reactor was measuredby a mechanical gauge and the temperature by an infrared pyrometer. Amolecular sieve was used to impede oil back-diffusion at the input ofthe pump. The working pressure was adjusted by varying the flow rate ofthe pump by using a control gate valve. The gas panel was classical,using ¼-inch stainless steel tubes. Flow rates were controlled by massflow control.

[0012] The reactor was purged with a hydrogen flow of 4 liters min-1,and the working pressure of 10-100 Torr was established by opening thegate valve that separated the pump and the reactor. The evacuation linethat was used at atmospheric pressure was automatically closed by theopening of the gate valve. The gas flow rates were measured understandard conditions, i.e., 1 atm and 20° C., even when the reactor wasat subatmospheric pressure.

[0013] The substrate can be GaAs, Si, Al203, MgO, SiC, ZnO, LiGaO2,LiAlO2, Cd Te, SiC, InAs, InP, Ga, Sb, InSb, MgAl204 or GaN. Preferably,GaAs is used as the substrate. The epitaxial layer quality is sensitiveto the pretreatment of the substrate and the alloy composition.Pretreatment of the substrates prior to epitaxial growth was thus foundto be beneficial. One such pretreatment procedure is as follows:

[0014] 1. Dipping in H2SO4 for 3 minutes with ultrasonic agitation;

[0015] 2. Rinsing in Deionized H2O;

[0016] 3. Rinsing in hot methanol;

[0017] 4. Dipping in 3% Br in methanol at room temperature for 3 minutes(ultrasonic bath);

[0018] 5. Rinsing in hot methanol;

[0019] 6. Dipping in H2SO4 for 3 minutes;

[0020] 7. Rinsing in deionized H2O, and

[0021] 8. Rinsing in hot methanol.

[0022] After this treatment, it is possible to preserve the substratefor one or two weeks without repeating this treatment prior to growth.

[0023] Growth takes place by introducing, metered amounts of thegroup-III alkyls and the group-V hydrides into a quartz reaction tubecontaining a substrate placed on an rf-heated susceptor surface. The hotsusceptor has a catalytic effect on the decomposition of the gaseousproducts; the growth rate is proportional to the flow rate of thegroup-III species, but is relatively independent of temperature between700° and 1000° C. and of the partial pressure of group-V species aswell. The gas molecules diffuse across the boundary layer to thesubstrate surface, where the metal alkyls and hydrides decompose toproduce the group-III and group-V elemental species. The elementalspecies move on the hot surface until they find an available latticesite, where growth then occurs.

[0024] High quality III-Iv materials may be grown in the method of thesubject invention by low pressure metallorganic chemical vapordeposition (LP-MOCVD). Other forms of deposition of III-IV films such asin the subject invention, may be used as well including MBE (molecularbeam epitaxy), MOMBE (metalorganic molecular beam epitaxy), LPE (liquidphase epitaxy and VPE (vapor phase epitaxy).

[0025] The layers of the heterostructure are grown by aninduction-heated horizontal cool wall reactor. Trimethylindium (TMI),Triethylgallium (TEG) and Trimethyl Arsenic (TMAs) are typically used asthe sources of Indium, Gallium, and Arsenic, respectively. TrimethylAluminum (TMAI) and Trimethyl Antimony (TmSb) are used as sources ofAluminum and Antimony, respectively. Sample is typically grown on asapphire substrate. A barrier layer of AlInSb or GaInSb is individuallylaid on the substrate at thicknesses from 50 Å to a few μm. The dopedactive layer may be InAsSb doped with an n-type dopant, such as SiH4.The example of optimum growth conditions for the respective layers arelisted in Table 1. The confinement of the active layer for the subjectinvention should be as a heterostructure.

[0026] Doping is preferably conducted with bis-cyclopentadienylmagnesium (CP2Mg) for p-type doping and silane (SiH4) for n-type doping.Doping is performed through a BCP2Mg bubbler with H2 as carrier gas andat temperatures from −15° C. to ambient temperatures at 20-1500 cm3min.-1 and onto either a hot or cooled substrate. Dilute SiH4 may besimply directed at ambient temperatures onto the hot, substrate at 20-90cm3 min. 1.

[0027] Dopants usable in the method of the subject invention are asfollows: n dopant p dopant H2Se (CH3)2Zn H2S (C2H5)2 Zn (CH3)3Sn (C2H5)2Be (C2H5)3Sn (CH3)2Cd SiH4 (ηC2H5)2Mg Si2H6 Cp2Mg GeH4

[0028] Codoping with two or more dopants may also be utilized.

[0029] In a preferred doping method for incorporating the maximum amountof p-type dopant on the layer, once the p-type layer to be doped isfully grown, the heat source is terminated and the substrate allowed tocool; the metal and hydride sources are terminated; the dopant flow, forinstance DEMg, is initiated at the temperatures indicated for diffusiononto the cooled substrate/epilayer which has been previously grown.After about 2-3 minutes, the dopant flow is terminated and the nextepilayer grown. By this method, it is found that 1020 atoms/cm3 of Mgmay be placed on the top surface of the epilayer. TABLE 1 Optimum growthconditions for representative III-IV materials. AlInSb GaInSb InAsSbGrowth Pressure 76 76 76 Growth Temperature 500 500 450 (° C.) Total H2Flow 3 3 3 (liter/min) TMI (cc/min) 25 25 50 TMAs — — 7 TMA1 3 — — TEGa— 4 — TMSb 45 45 40 Growth Rate 150 150 250 (Å/min)

EXAMPLE

[0030] The epitaxial layers are grown on (001) GaAs substrates using ahorizontal flow low pressure metalorganic chemical vapor deposition(LP-MOCVD) reactor. The inductively heated SiC-coated graphite susceptoris rotated at a speed of ˜50-100 rpm to achieve better uniformity films.Trimethyl-gallium (TMGa) and triethyl-gallium (TEGa) are used as thegallium (Ga) source materials; trimethyl-indium (TMIn) is used as theindium (In) source and Trimethyl Arsenic (TMAs) is used as a source ofArsenic. Trimethyl Aluminum (TMAI) and Trimethyl Antimony (TmSb) is usedas the source of Aluminum and Antimony, respectively.Bis-cyclopentadienyl-magnesium (Cp2Mg) and silane (SiH4) are used as themagnesium (Mg) and silicon (Si) doping source materials respectively.The carrier gases include Palladium diffused hydrogen and resin purifiednitrogen.

[0031] The device structure is shown in FIG. 1. In a preferredembodiment, a thin (0.5 μm to 10 μm and preferably 5000 Å) AlInSbbarrier is grown at low temperature (500° C.) on the GaAs substrate.This layer is not doped. Then, a 0.1 μm to 5 μm and preferably 3μm-thick n-doped InAsSb active layer (preferably doped with Si) is grownat 450° C. at a growth rate of 1.5 μm/hr using TMI, TMAs, TMSb, SiH4 andhydrogen as the carrier gas. The InAsSb layer has a different bandgapthan AlInSb, however the respective layers are lattice matched. Thislayer typically exhibits a room temperature free electron concentrationof 2×1018 cm-3 and mobility of 30,000 cm2/Vs. Then, the growthtemperature is maintained at 500° C., while growing p-doped AlInSb(preferably doped with Mg) is grown using TMAI, TMIn, TMSb and Cp2Mg ata growth rate of 1.0 μm/hr, again, lattice-matching the two layers. Thesample was then slowly cooled down to avoid formation of cracks.Further, the active layer may be InBiSb, InTiSb, InAsBiSb, or InPBiSb.

[0032] While it is not necessary that the upper contact layer be of thesame material as the lower contact layer, it is necessary that eachcontact layer have a bandgap different from the active layer, while eachlayer is lattice matched to the adjacent layer.

[0033] After epitaxial growth, the wafers are annealed using rapidthermal annealing (RTA) under nitrogen ambient for 30 seconds at 500° C.to achieve low resistivity p-type AlInSb. Typically, the roomtemperature free hole concentration is 1×1017 cm-3 and mobility is 500cm2/Vs. Ni/Au metal contacts are deposited on the upper layer of p-typeAlInSb using an electron-beam evaporator. Ti/Au metal contacts are thendeposited on the lower layer of undoped AlInSb using an electron-beamevaporator. The metal contacts are defined by a conventional lift-offprocess known in the art.

[0034] The structure requires only two contacts and utilizes simplefabrication techniques. With proper control of material compositionduring the growth of these structures, lattice matched layers withdifferent bandgaps result which leads to detectors with highresponsively at two or three wavelength regions. In addition to AlInSbas a barrier material GaInSb may also be used. The resultingphotoresponse is shown in FIG. 3. Note that the photoresponse at longerwavelengths changes drastically with an applied bias.

[0035] In addition to the structure shown in FIG. 1, a doubleheterostructure could be used with a wide gap top layer which providesbetter carrier confinement which leads to better photoresponse, and alsocould be used for a third wavelength range detection.

[0036] A possible third type of structure is an npn (or pnp)phototransistor. This is accomplished by simply alternating the dopingof the grown layers to result in a transistor device. The device isstill a two terminal device, with base current being supplied by thephotogenerated carrier. The main advantage of a phototransistorstructure is the possible current gain as a result of transistor action.

[0037] While the invention has been described with reference to apreferred embodiment, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments and equivalents.

What is claimed is:
 1. A multi-color photodetector device comprising aheterostructure with a substrate, a lower contact layer, a single activeregion and an upper contact layer, each of said upper and lower contactlayers having a bandgap; said active region having a bandgap differentfrom the bandgap of said lower and upper contact layers; and said activeregion being lattice-matched to each of said upper and lower contactlayers.
 2. The device of claim 1, wherein said upper and lower contactlayers are AlInSb or GaInSb.
 3. The device of claim 1, wherein saidactive region is InAsSb, GaInSb, InBiSb, InTlSb, InPsb, InAsBiSb,InPBiSb.
 4. The device of claim 1, wherein said substrate is GaAs, InP,or Si.
 5. The device of claim 1, wherein said upper contact layer isp-type doped.
 6. The device of claim 1, wherein said active region isn-type doped.
 7. The device of claim 2, wherein said upper contact layeris doped with Mg, Zn, C, or Be.
 8. The device of claim 3, wherein saidactive region is doped with Si.
 9. The device of claim 1, wherein saidlower contact layer is about 0.1 μm up to 10 μm thick and said activeregion is about 0.1 μm up to 5 μm thick.
 10. A method of preparing amulti color detector device, comprising the steps of: a) growing a lowercontact layer having a first bandgap on a substrate; b) lattice matchingand growing a single active layer or a multi quantum well having asecond bandgap different from said first bandgap on said lower contactlayer; c) lattice matching and growing an upper contact layer having athird bandgap different from said second bandgap on said active layer toform a heterostructure; and d) annealing the heterostructure.
 11. Themethod of claim 10, wherein said upper and lower contact layers areAlInSb or GaInSb.
 12. The method of claim 10, wherein said active regionis InAsSb or GaInSb, InTlSb, InBiSb, InBiAsSb, or multiquantum well ofAlSb, InAs, GnSb.
 13. The method of claim 10, wherein said substrate isGaAs, InP, GaSb, InSb, InAs, Si, Al2O3, SiC, or CdTe.
 14. The method ofclaim 10, wherein said upper contact layer is p-type doped.
 15. Themethod of claim 10, wherein said active region is n-type doped.
 16. Themethod of claim 11, wherein said upper contact layer is doped with Mg,Be, Zn, Cd, C or codoping.
 17. The method of claim 12, wherein saidactive region is doped with Si, Se Te, Ge, S, or codoped.
 18. The methodof claim 10, wherein said lower contact layer is about 5 μm to 10 μmthick and said active region is about 0.1 μm to 5 μm thick.
 19. Amulti-color detector device comprising a heterostructure with asubstrate, a lower contact layer of AlInSb or GaInSb, a single activeregion of InAsSb InTlSb, InBiSb, InPSb, or InBiAs doped with a n-typedopant, and an upper contact layer each of said upper and lower contactlayers having a band gap; said active region having a bandgap differentfrom the bandgap of said lower and upper contact layers; and said activeregion being lattice-matched to each of said upper and lower contactlayers.
 20. The device of claim 19, wherein said upper contact layer isdoped with Mg, Be, Zn, Cd, C or codoped.
 21. The device of claim 19,wherein said active region is doped with Si, Se Te, Ge, S, or codoping.22. The device of claim 19, wherein said lower contact layer is about 5μm to 10 μm thick and said active region is about 0.1 μm to 5 μm thick.23. A method of preparing a two color detector device, comprising thesteps of: a) growing a lower contact layer of AlInSb or GaInSb having afirst bandgap on a substrate of GaAs; b) lattice matching and growing asingle active layer or multiquantum well of AlInSb or GaInSb having asecond bandgap different from said first bandgap on said lower contactlayer at a temperature of about 450° C.; C) doping said active layerwith n-type dopant; d) lattice matching and growing an upper contactlayer having a third bandgap at a temperature of about 500° C. differentfrom said second bandgap on said active layer to form a heterostructure;e) doping said upper contact layer with a p-type dopant, and f)annealing the heterostructure.
 24. The method of claim 23, wherein saidupper contact layer is doped with Mg, Be, Zn, Cd, C or is codoped. 25.The method of claim 23, wherein said active region is doped with Si, SeTe, Ge, S, or is codoped.
 26. The method of claim 23, wherein said lowercontact layer is about 0.1 μm to 10 μm thick and said active region isabout 0.1 μm to 5 μm μm thick.